CRYPTOPHYTA 325

Fig. 9.5 (a) General organization of an ejectisome showing the two subparts. (b) A model of an ejectisome being fired outside of the cell. (c) A drawing of a discharged ejectisome. (After Hovasse et al., 1967.)

(a)		(b)		(c)

Ecology

In comparison with other algal groups, the Cryptophyta appear to be especially light sensitive, often forming the deepest living populations in clear oligotrophic lakes (Nauwerk, 1968). In higher mountain and north temperate lakes, cryptomonads and other flagellates are present in the water column throughout the winter. Because of the low light intensity under snow and ice cover, these algae concentrate in surface waters to receive sufficient light from net photosynthesis (Wright, 1964; Pechlauer, 1971). Survival at these extremely low light levels depends not only on a highly efficient photosynthetic system, but also on slow rates of cell respiration at low water temperatures and reduced winter zooplankton grazing. In spring, with the disappearance of snow and resulting sudden increase in light in Arctic and mountain lakes, cryptomonads suffer considerable light stress, such that the biomass maximum moves to deeper waters (Kalff and Welch, 1974).

Cryptophytes will often undergo diel vertical migrations with an amplitude less than 5 meters. In small humic forest lakes, species of Cryptomonas are positively phototactic in the morning, moving into the phosphorus-depleted upper layer. Later in

the day the cells move away from the uppermost water layer, avoiding high levels of irradiance, and move into the phosphorus-rich hypolimnion (Knapp et al., 2003). A further advantage of this cycle is the reduction of grazing pressure by zooplankton (for which cryptophytes are a preferred food) (Loret et al., 2000) which often migrate in the reverse direction.

Cryptophyte algae are mixotrophic, capable of phototropy and phagotrophy. Phagocytotic ingestion of bacteria is thought primarily to provide a source of phosphorus and nitrogen in nutrientlimiting conditions (Urabe et al., 2000). These algae are also chemotactic, swimming in a straight line until they reach a nutrient patch, at which time the cells stop and tumble in the volume of high-nutrient concentration (Lee et al., 1999).

Cryptophytes are the dominant algae in the freshwater lakes of Antarctica. The best studied lakes are those in the McMurdo Dry Valleys (Fig. 9.6) (Roberts and Laybourne-Parry, 1999; McKnight et al., 2000). The McMurdo Dry Valleys are the largest ice-free areas in Antarctica (about 4000 km2) and constitute a polar desert with temperatures ranging from 45 C to 5 C. These valleys remain ice free because the Trans-Antarctic Mountains not only block the flow of the ice sheet, but also block the flow of moisture, the valleys